Cheetah

I converted a Lidl 'hand throwing' glider to powered RC last year and with some aerodynamic 'improvements' it flies and glides really quite well. It weighs 225 g.

Although reasonably efficient it does not exactly look like a glider with rather 'chunky' wings.

I then noticed the Cheetah hand throwing glider. Similar in concept to the Lidl, slightly smaller and cheaper but with a rather more 'glider like' appearance.

At 890 mm span it looks to have a limited wing area for a electric RC conversion.

Quite nicely moulded, actually slightly better than the Lidl, but the 'clip together' two piece wing could present a strength issue as the glider only weighs 119 g so a conversion close to 250 g would double the wing loading.

Not really in the 'spirit' of a conversion but one solution would be to add a substantial wing centre section. This would increase the wing area and aspect ratio yet without adding significantly to the load on the existing foam wings.

Looking through various forums it seem the Bolt glider has indeed been successfully converted to powered RC. Your comment on the strength of the Cheetah wings has also been repeated and even brought into question the practicality of doing so as apart from the control surfaces being market out everything else requires 'digging out' of foam to fit.

The strength of the wing brought to mind the Antonov 225. It uses the wings from the AN124 with a centre section added (and 2 more engines) which enables the 225 be 50% heavier yet with only a 2% increase in wing loading.

So a centre section with 1/3 more area would allow the Cheetah's weight to rise to 160 g without increasing the wing loading. Furthermore some of the bending arises from its centre clip together joint which a one piece wing centre section would not, so my hope is the existing foam wings without specific reinforcement will be able to handle at least a 200 g all up weight.

My experience using thin Depron suggested the Cheetah wing centre section would be strong enough without any spar just relying on the 2mm skin. All it would need would fairly close spaced ribs and a shear web.

As my supply of 2 mm Depron is limited, irreplaceable and cutting ribs tends to wasteful of sheet I decided to 'print' the ribs as well as the intervening shear web sections.

Each unit weighs 0.6,g.and is simply repeated for as many ribs as required

They are glued one at a time to the Depron skin under surface.

Although only intended to take the shear forces when all the webs are glued together in a line they will also act as a 'spar' hopefully adding further stiffness.

Each foam wing panel will be trimmed to plug into the centre section so when glued in place the bending loads are transferred directly into the Depron skin.

As the aileron servo wires will have to run inside the centre section only the top skin as far as the shear web is glued on. This make the centre section rigid enough to handle.

The RH foam wing added to the centre section.

Although the Cheetah wing trailing edge is reasonable with Depron it is possible to get much finer so trailing edge extensions made from two 2 mm Depron layers which are sanded down on their inner sides before being glued together. This leaves the smooth stronger Depron skin intact right down to the very edge.

A 'before and after' comparison.

There is an argument that a 'cut off' trailing edge has little or no penalty as any loss in aerodynamic efficiency is countered by the reduction in skin drag. I am not so sure for a glider. Anyway in this case the extra wing area will be useful.

Before fixing the wing on it seemed logical to 'excavate' the cockpit area and the canopy for the battery, rx and ESC.

I even kept all the excavated 'spoil' to see how much weight was saved. Bulky but foam is very light so just under 3 g at the moment although the excavation might have to be enlarged depending on the size and final position of the battery.

The canopy is secured by a small printed peg and matching socket at the front.

With a pair of small magnets (also in printed housings) at the rear.

Its secure enough to survive a 'look no hands' test.

All this took many hours to do but with the lock down in place it seemed a reasonable way to pass the time..

With the aileron servo wires run inside the centre section and out through the leading edge at the centre the wing skin can be completed and be permanently glued in.

The enhanced Cheetah is now in its final configuration.

The next task is to establish the thrust that is available from the small Racestar BR1504 motor with its recommended 5 x 3 prop. To do this I printed a simple test stand.

On a 3 s it drew 4.9 A showing close to 50 W . At its target weight of 200 g that will give 114 W/lb which should be quite adequate.

Next the fuselage nose was cut back to a suitable point and a printed motor mount created to provided the locations to skin in Depron from the oval fuselage to the 1" circle for the folding prop spinner..

It will look something like this test piece.

In the final version it will actually be build around the permanently installed motor.

I need the 370 mAh 3s battery to establish whether the position of the elevator servo has to contribute to achieve likely CofG..

I was concerned that inside the cockpit the 10A ESC would be very well insulated and might get hot, particularly as it is sharing a tight space with the battery so outside it had to be.

As a concession to drag its heat shrink was removed and it was placed in a printed 'holder' submerged almost flush with the outer skin.

The wires pass straight through into the cockpit.

Notice the terrifyingly small JST ZH connector that is required by the micro rx.

The 370 mAh 3s is retained by the printed clips.

At this stage it was possible to determine if the position of the elevator servo would be significant to achieve a suitable CofG. It was not so it could be placed under the wing and far enough back so its pull/pull wires would not interfere with a hand launch.

To achieve light weigh 5 lb mono filament line was used.

Slowly getting there.

Just as well there is nothing else to do with the Covid 19 'lock down'.

The prop is a HK 5 x 3 folder with a printed spinner but unfortunately one blade is broken and HK have been out of stock for some time.

I found a 6 x 3.5 printed blade on Thingyverse.

The root shape is all wrong for the HK hub unit but the blade itself looks reasonable. Of course it is only available as a STL file that cannot be edited so with a bit of CURA 'fiddling' it was possible to create a blade of the correct root thickness and hole diameter so it only needed some work with files to create a blade that fitted the HK hub unit. To avoid over loading the motor it was cut down to 5" diameter.

Works and folds well enough but for the maiden (whenever that may be!) I will substitute a fixed 5x3 as it generates a bit more thrust.

All this just because I wanted a cheap foamy RC conversion that looked a bit "nicer" than a cheap foamy Lidl conversion.

With a 500 mAh 2s and a HK folding 5 x 3 it draws 2.7 A showing 20 W on the Watt meter. It now weighs 184 g which, if I have done the sums right, gives 50 W/lb. Not spectacular but hopefully an adequate performance.

As the bulkhead had to be replaced I took the opportunity to add some motor cooling ducts.

Inlet above the spinner.

And cheek outlets on both sides.

The cheeks do limit the degree of prop folding a bit so now perhaps its a bit more 'chipmunk' than 'Cheetah'.